1. Field of the Invention
The present invention relates to apparatus for forming a colored band of graded intensity having a curved cut-off line along one edge thereof and a desired transverse pattern of graded intensity along a longitudinally extending area of a workpiece in the form of a fexible sheet of plastic interlayer material suitable for use as an interlayer in laminated safety glass. The workpiece is preferably coated by electrostatic spraying to insure that very fine droplets of dye composition form a band of color having a desired intensity pattern along its transverse dimension that includes a first coating portion of substantially uniform maximum intensity merging into a second coating portion of changing intensity that merges into a third coating portion of finely reduced intensity that merges into a barely perceptible cut-off line between coated and uncoated portions.
This invention is especially concerned with the partial coating of flexible interlayer material that is a component of laminated glass used in automobiles. The partial coating according to the present invention is performed in such a manner that it is unnecessary to differentially stretch partially coated interlayer sheets removed from a continuous ribbon to provide a curved cut-off line along the edge of the colored portion so that when the partly dyed interlayer is laminated to one or more curved sheets of rigid transparent material, such as glass or a recognized rigid transparent plastic substitute for glass such as polycarbonates, acrylic plastics, polyesters and rigid polyurethanes (hereinafter also referred to as glass), the cut-off line between the coated and uncoated portions will appear to have a horizontal line in the curved laminated windshield installed in an inclined relation in an automobile frame.
In electrostatic spraying of sheets of interlayer material as practiced by the prior art, a non-conductive workpiece is exposed to highly charged particles of a dye composition. A grounded shield of electroconductive material is interposed between an electrostatic spray gun and the workpiece and a manifold is located behind the outboard edge of the grounded shield and between the lower surface of the shield and the upper surface of the workpiece to provide a positive pressure of a non-reactive fluid, such as air, in the direction of the edge of the grounded shield parallel to the surface of the workpiece being coated so as to reduce the intensity of underspray that is deposited on the upper surface of the workpiece in facing relation to the shield. Grounding the shield selectively attracts many highly charged particles of dye contained in the dye composition that would otherwise mutually repel one another and deposit on the upper surface of the workpiece in the region over a wide area facing the shield and extending laterally from its inboard edge.
Nevertheless, in large scale production runs, liquid spray composition accumulates on the lower surface of the shield to such an extent that droplets of the accumulated spray composition larger than the particles of the electrostatic spray fall on a workpiece to spoil its optical properties where the droplets form. Furthermore, despite the force of manifold air, particles of spray composition tend to accumulate on the manifold, which faces an area of the workpiece desired to be free of coating. When the accumulation builds up sufficiently during a production run, a thin, second band of color is imparted to the workpiece along a line facing the manifold in a region desired to be uncoated.
The concept of applying paints and other tinting compositions by an electrostatic spray system has been developed. U.S. Pat. No. 3,645,477 to Cowen discloses an air atomized electrostatic spray device in which air is supplied to the device for the purpose of atomizing a liquid to be sprayed. The air is also employed for operating a self-contained electrogasdynamic power generator for charging the atomized coating material and for maintaining an electrostatic depositing field having one terminus adjacent the locus of atomization. A highly charged atomized mist is imparted to a substrate to be coated. Electrostatic spraying develops a coating of a given intensity characterized by a low transmission coefficient more rapidly than older spraying methods.
The mist so produced comprises highly charged particles that mutually repel one another during the electrostatic spray process. This mutual repulsion causes the particles to spread over a wide area en route to a substrate to be coated. Hence, the electrostatic spray process is suitable for coating an entire substrate uniformly. When an electrostatic spray is applied to an exposed portion only of a substrate, the mutual repulsion of the spray particles causes the electrostatic spray particles to deposit on a large area outside of the exposed portion to which the electrostatic spray is applied and form a pattern of gradually reducing intensity within said large area outside of said exposed portion. Even the interposition of a mask or shield, which controls the boundary in operations in which hydrolyzable salt compositions and sprays other than electrostatic sprays are applied to form coatings in the form of a band, fails to avoid extensive areas of fade-out in coatings applied by electrostatic spraying.
Since the details of the electrostatic spray device does not form part of the present invention and such devices are readily available commercially, the details of the spray gun or power generator for use with the spray gun will not be described in detail in this specification. However, details of a suitable electrostatic spray device may be found in U.S. Pat. No. 3,645,447 to Cowen and details of a suitable electrogasdynamic generator for such a suitable electrostatic spray device are recited in U.S. Pat. No. 3,651,354 to Cowen. The disclosures of these patents relative to an electrostatic spray device and an electrogasdynamic generator for such a device are incorporated herein by reference in order to avoid an excessively long specification.
When the cut-off line between the coated and uncoated portions of a partially dyed interlayer sheet incorporated in a shaded laminated windshield is too abrupt, an optical illusion in the form of a line parallel to and spaced from the curved cut-off line of the shaded laminated windshield becomes visible. Such a line of optical illusion is hard to avoid when the colored band is applied by printing or by spraying using a mechanical shield interposed in the path of the spray. Interposing a manifold in spaced relation behind the edge of a shield and blowing a gentle flow of air or other non-reactive fluid between the shield and the sheet to be coated improves the ability to avoid the optical illusion, particularly when the spray is applied electrostatically and the shield is composed of electroconductive material and is grounded. The particles of an electrostatic spray are highly charged and tend to repel one another. This mutual repulsion tends to spread the cut-off portion of the resulting coating over a wide area. A grounded shield attracts a portion of the highly charged particles, thus reducing the intensity of the coating that forms in the cut-off portion of the coating, and providing a more gradual change in intensity near the cut-off line.
2. Description of the Prior Art
U.S. Pat. No. 2,676,114 to Barkley discloses various methods of producing graded coatings by vacuum evacuation from a line of several evaporation sources disposed in alignment with the direction of gradation of coating using one or more shields interposed between a line of evaporation sources and the workpiece to be coated. Such coatings develop sharp lines of demarcation between coated and uncoated areas and between adjacent areas exposed to different numbers of evaporation sources and between adjacent areas shielded by a different number of shields interposed between the sources and the workpiece. This technique uses mechanical barriers to apply graded coatings to glass sheets.
U.S. Pat. No. 3,004,875 to Lytle discloses applying a band of metal salt composition at an oblique angle toward an edge portion of a substrate to be coated, using a shield to limit the area of application of the coating composition. The resulting band is of gradually increasing intensity from its boundary with the uncoated portion and the edge of the substrate containing the band. Furthermore, the resulting band has a mottled appearance, particularly in its boundary portion adjacent to the edge of the shield because of eddies that evolve beneath the shield near the edge of the shield during the coating operation.
U.S. Pat. No. 3,113,034 to Fix discloses applying a band of coloring material having a repetitive pattern of curved cut-off line to a continuous sheet of flexible plastic interlayer material using a roller coater to which dye is imparted for imparting said dye to a surface of the sheet while the roller coater simultaneously rotates and reciprocates axially against said sheet while the latter moves past the roller coater. While this principle of transferring dye has been established, the process has not completely eliminated some residual blur in the applied dye pattern due to the rubbing action of the roller against the flexible plastic interlayer material so that the cut-off line between dyed and undyed portions appears fuzzy.
U.S. Pat. No. 3,078,693 to Lytle discloses applying a metal salt composition to a portion of a hot glass sheet to form a metal oxide coating and then laminating the coated glass sheet to one side of a plastic interlayer. The metal oxide films have a mottled appearance which is not removed by lamination.
U.S. Pat. No. 2,088,542 to Westin impinges one or more currents of gas into an atomized spray of coating composition at a point closer to the substrate to be coated then to the point of origin of the jet to delimit the coated area from the uncoated area. A sharp line of demarcation results between the coated area and the uncoated area.
U.S. Pat. No. 2,082,182 to Schacht uses a shield to confine the flow of pulverulent particles toward a substrate and forms a sharp line of demarcation between the coated and uncoated portions of the substrate.
U.S. Pat. No. 1,861,475 to Hopkins and Odgen uses compressed air to confine the application of a spray of lacquer. The sprayed portion is sharply separated from the unsprayed portion.
U.S. Pat. No. 2,283,253 to Haven controls the width of a line of metal formed by spraying metal particles upon a glass substrate at an oblique angle thereto by providing a guard member positioned between the spray source and the substrate. The spray, on passing the edge of the guard member, causes the formation of eddies that set up back sprays, which form a metal mist upon the glass beyond the area desired to be coated. Gaseous fluid is provided under pressure beneath the guard member to counteract the formation of the eddies and prevent the metal particles from passing beneath the guard member. The resulting boundary between the coated line of opaque metal and the uncoated transparent portion is sharp and abrupt, not gradual as desired for purposes of the present invention, even though the boundary portion of metal may be tapered.
U.S. Pat. No. 3,305,336 to Browne et al discloses a spray gun system for applying a film on a glass surface. The film is formed on a band having a major area of maximum intensity bordered by a fade-out line that defines a fade-out area in which the transmittance of the film gradually and progressively increases until its value reaches the clarity of the glass substrate. This patent refers to conventional spray techniques rather than electrostatic spray techniques.
None of the aforesaid patents relate to applying a colored coating having across its transverse dimension a main relatively wide first coating portion of essentially uniform maximum intensity that merges into a second coating portion of graded intensity which, in turn, merges into a third, relatively narrow, boundary coating portion of feathered intensity by electrostatic spraying. None of the aforesaid patents was confronted with the problem of having to alter a selected portion of a uniform spray pattern resulting from the mutual repulsion of electrostaticlly charged particles into a portion of graded intensity.
A copending patent application of Dennis S. Postupack, U.S. Ser. No. 868,013, filed Jan. 9, 1978, now U.S. Pat. No. 4,138,284, for Method of Forming Graded Shade Band on Substrate, discloses a method and apparatus for applying a shade band by electrostatically spraying a dye composition past a grounded electroconductive shield and using a combination of a single grounded shield and a manifold between the shield and the substrate to control the location of the cut-off line between the coated and uncoated regions of the substrate and to limit the amount of dye deposited in the vicinity of the cut-off line. While this system was successfully used to produce a coating having a finely graded cut-off portion of diminishing intensity such as would be capable of avoiding a line of optical illusion parallel to the cut-off line, further developments are needed to produce a more complicated pattern of gradation that incorporates the finely graded cut-off line between coated and clear portions.